The present invention relates to an air venting belt used in curing a retread tire by the precured retreading method in which a worn-out tread is removed from a used tire by bufffing, a sheet-like unvulcanised cushion rubber is applied thereon, a belt-like precured tread is applied thereon, and the cushion rubber is vulcanised, thereby integrating the buffed tire and the precured tread.
Heretofore, an air venting belt has been used in the method which comprises a heat resistant and non-expansible nylon cloth.
In general such a process for retreading a tire, as shown in FIG. 12 is as follows. A belt-like precured tread b is applied over the crown of a buffed tire c (the tire to be retreaded from which the worn-out tread has been buffed) which has been covered with a non-vulcanised sheet-like cushion rubber d. Thereafter, the assembly is covered with an envelope e (a rubber cover) which protects the outer surface of the tire from direct contact with the curing steam. The overall appearance of the envelope e is like a doughnut having a "U"-shaped cross section open inwardly. At this time, strips of air venting belt a made of nylon cloth are laid between the tread and the rubber cover at prescribed intervals in the circumferential direction. Thus prepared tires g are then loaded with others into a vulcanising vessel with seal plates f laid between the tires and heated steam, pressure of which is several kg/cm.sup.2 is supplied to both the tread and sidewall zones as shown by arrows P. At this time, since the above-mentioned belts a are laid between the tread b and the envelope e, air between the tire and the envelope vents through the mesh or open weave of the cloths as shown by arrows A.
However, as mentioned above, since a non-expansible belt a is used in the conventional method, the tire grooves h are unusually deformed, as shown in FIG. 13. That is to say, the non-expansible belt a prevents the envelope from stretching and fitting itself to the inner surface of the tire grooves h. Therefore, the force shown by arrows F operates on the end faces of the tread b, and a stress concentration happens in a narrow or thin portion i located at the bottom of the tire grooves h, and deformation happens. For example the width of the groove becomes extremely narrowed (see the tire groove h in the upper side of FIG. 13), or even the groove takes a "W"-shaped bottom face because of the association to a bend of the cushion rubber (see the tire grooves h in the lower side of the FIG. 13). As mentioned above, the conventional air venting belt thus causes a bad appearance for the tire grooves h and reduces the value and quality of the tire.
Further, since several tires g are loaded for vulcanisation, pressed together as shown in FIG. 14, in the portion where the belt is positioned marking from the fabric may occur and cause a bad appearance.